Power supply circuit

ABSTRACT

A power supply circuit which comprises a first power source terminal for applying a plus voltage and a second power source terminal for applying a minus voltage. A first transistor is inserted between the first power source terminal and a load. A second transistor is inserted between a base of the first transistor and a ground potential and is controlled by the voltages applied to the first and second power source terminals. A capacitor is connected to the second power source and ground potential. A sequence of applying or cutting-off voltages to be applied to the load is predetermined even when a sequence of applying or cutting-off the voltages from said first and second power source terminals becomes erratic.

BACKGROUND OF THE INVENTION

The present invention relates to a power supply circuit, and more particularly to a voltage selector circuit in the case of operating drive (or measured) means with a plurality of power sources.

In a magnetic recording/reproducing apparatus etc., a plurality of power sources of, e.g., +12 V and +5 V have heretofore been used in a circuit for moving the magnetic head of a magnetic disk device and for pressing the magnetic head against a magnetic disk, etc.

To this end, two voltage sources have been employed. When the respective voltages rise due to the turning ON or OFF of a power supply, writing or reading errors might occur depending upon whether the circuit for pressing the magnetic head operates earlier or later.

In the case of driving or measuring a field-effect transistor (FET), such as a GaAs FET or the like, with a plurality of power sources, a first voltage source connected to the drain of the FET and a second voltage source connected to the gate thereof must be applied or cut off in a predetermined sequence. More specifically, in turning off the power supply, the second voltage source is turned "on" to apply a bias voltage to the gate electrode of the FET, and the first voltage source is subsequently turned "on" to apply a bias to voltage the drain electrode thereof. In turning on the power supply, the bias voltage applied to the drain electrode is rendered "off" and thereafter, the bias voltage applied to the gate electrode is rendered "off." Otherwise, the FET will breakdown. In order to prevent breakdown of the FET, switching operations at the turning off or turning on of the power supply have been scrupulously performed. Alternatively, a sequencer circuit has been assembled of a relay, etc., so that the respective voltages may rise or fall in a predetermined sequence when the power supply is turned off or turned on. The addition of the relay, etc., to the power supply circuit of this type, however, incurs the disadvantages of being a large-sized power supply device, having low circuit reliability and having a high cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a power supply circuit which is free from the disadvantages stated above.

The present invention is a power supply circuit in which, even when the sequence of applying or interrupting voltages from a plurality of voltage sources becomes erratic, it is ensured that a first or second driving (or measuring) element for the drive (or measured) means is operated after the second or first driving (or measuring) element has been operated.

The present invention comprises a power supply circuit having

a first power source terminal;

a second power source terminal having an applied voltage opposite in sign to a voltage applied to the first power source terminal;

a first switching element which is inserted in series between the first power source terminal and a load;

a second switching element which is inserted between a base of the first switching element and a reference potential and which is controlled by the voltages applied to the first and second power source terminals; and

a capacitive element which is connected between a second power source and the reference potential, control voltages being applied to the second switching element from the first and second power sources, so that a sequence of applying or interrupting voltages to be applied to the load is predetermined even when the sequence of applying or interrupting the voltages of the first and second power source terminals becomes erratic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is circuit diagram of a power supply circuit of an embodiment of the present invention;

FIG. 1B is an amplifying circuit to be connected to the output terminals of the power supply circuit of FIG. 1A;

FIG. 2 is a circuit diagram of a second embodiment of the present invention;

FIG. 3 is a circuit diagram of a third embodiment of the present invention;

FIG. 4 is a circuit diagram of a fourth embodiment which is similar to the embodiment of FIG. 1, but to which a pulse supplying circuit in FIG. 2 and a voltage regulator circuit in FIG. 3 are added;

FIGS. 5A to 5D are waveform diagrams for explaining the operations of the power supply circuit in FIGS. 1 to 4; and

FIGS. 6A to 6E are waveform diagrams for explaining the operations of the power supply circuits in FIGS. 2 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1A shows the fundamental circuit arrangement of the present invention. Terminals T₁, T₂ and T₃ are input terminals. A voltage on the order of +V_(in) =+15 V is applied across the plus input terminal T₁ and the reference or constant potential terminal, for example, the ground terminal T₃, while a voltage on the order of -V_(in) =-6 V is applied across the minus input terminal T₂ and the ground terminal T₃.

Terminals T₄, T₅ and T₆ are output terminals. The voltage of the plus output terminal T₄ relative to the ground terminal T₆ is applied to, e. g., a drain exhibiting a comparatively low impedance, while the voltage of the minus output terminal T₅ relative to the ground terminal T₆ is applied to, e. g. a gate exhibiting a comparatively high impedance.

FIG. 1B shows an example of an amplifying circuit to be connected to the output terminals T₄, T₅ and T₆ of the circuit arrangement shown in FIG. 1A. The minus output voltage -V_(out) is supplied from the terminal T₅ and is divided by resistors Ra₁ and Ra₂, and resistors Rb₁ and Rb₂, thereby providing the gate bias voltages of GaAs field effect transistors Ta and Tb, respectively. The plus output voltage +V_(out) is applied to the drains of the transistors Ta and Tb through impedance elements Za and Zb, respectively. When the a.c. input signals V_(ina) and V_(inb) are applied to the amplifying circuits, the input signals are amplified through the transistor Ta and impedance element Za, and the transistor Tb and impedance Zb, respectively, thereby producing the output signals from the amplifying circuit through coupling condensers Ca and Cb.

An impedance component or a d.c. equivalent circuit of the amplifying circuit of FIG. 1B is Shown in FIG. 1A by the resistors having the value of 2Ω and 300Ω. In FIG. 1A, the resistor of 2Ω represents the impedance between the drains of the transistors Ta and Tb and ground. The resistor of 300Ω represents the impedance between the gates of the transistors and ground.

The plus input and output terminals T₁ and T₄ are connected through the collector-emitter path of a first transistor Tr₁, the base of which is connected to the ground potential through the collector-emitter path of a second transistor Tr₂. A resistor R₅ is connected between the base and collector of the first transistor Tr₁. A series circuit comprising resistors R₁, R₂ and R₃ is connected between the plus input terminal T₁ and the minus input terminal T₂, and a resistor R₄ is connected between the node of the resistors R₂ and R₃ and the base of the second transistor Tr₂, so that the plus input voltage +V_(in) is applied to the base of the second transistor Tr₂ through the resistors R₁, R₂ and R₄ and the minus input voltage -V_(in) is applied thereto through the resistors R₃ and R₄.

A Zener diode D₁, for setting a reference voltage, is connected between the node of the resistors R₁ and R₂ and ground. Further, the minus input terminal T₂ is connected to the minus output terminal T₅ through a first diode D₂. Still further, a second diode D₃ and a capacitor C₁ are respectively inserted and connected between the corresponding terminals of the first diode D₂ and ground.

The operation of the circuit arrangement in FIG. 1 will be described with reference to wave forms depicted in FIGS. 5A-5D. When the d.c. voltage +V_(in) as shown in FIG. 5A is first applied from a first voltage source (not shown) to the plus input terminal T₁, the second transistor Tr₂ has its base supplied with the plus voltage through the resistors R₁, R₂ and R₄ and therefore is in the "on" state. The base potential of the first transistor Tr₁ becomes a low potential, and is therefore in an "off" state.

When, in the above state, the minus voltage -V_(in) is applied from a second voltage source (not shown) as depicted in FIG. 5B, the minus output terminal T₅ is supplied with the minus output voltage through the diode D₂ without a time delay as seen from the falling edge 1 in FIG. 5D. On account of a voltage drop across the diode D₂, the voltage -V_(in) =-6 V applied to the minus input terminal T₂ appears as the minus voltage -V_(out) =-5.3 V at the minus output terminal T₅. At this point in time, the second transistor Tr₂ which is in the "on" state has the minus voltage -V_(in) applied to its base through the resistors R₃ and R₄ and is therefore inverted into an "off" state. Then, the first transistor Tr₁ is supplied with the plus voltage +V_(in) through the resistor R₅ and turns "on", so that the plus voltage + V_(out) =14.3 V is delivered to the plus output terminal T₄ as the plus input voltage +Vin =15 V. A rising waveform in this state is depicted by a rising edge 2 in FIG. 5C. In this case, a time delay τ, which is determined by the stray capacitances of the first and second transistors Tr₁ and Tr₂ and the resistances of the bias resistors, occurs.

As thus far described, the minus voltage -V_(out) is provided at the minus output terminal T₅, to which the gate electrode of an FET or the like is connected, immediately without any time delay or simultaneously with the application of the second voltage source. Accordingly, the gate electrode is supplied with the voltage.

Subsequently, the plus voltage +V_(out) is provided to the plus output terminal T₄, to which the drain electrode of the FET or the like is connected, with the time delay τ. Even when the plus voltage +V_(in) has been applied, the first transistor Tr₁ is in the nonconductive state and delivers no output voltage to the plus output terminal T₄. In this manner, irrespective of the sequence of applying the voltages from the first and second voltage sources, the output voltages with the time delay and in the desired sequence are provided to the minus output terminal and the plus output terminal without fail.

Now, a case of interrupting the voltages from the first and second voltage sources will be described with reference to FIGS. 5A-5D.

Let's consider a case where, in the state in which the plus voltage +V_(in) and the minus voltage -V_(in) depicted in FIGS. 5A and 5B are applied, the latter voltage applied to the minus input terminal T₂ is interrupted. In this case, the minus voltage applied to the base of the second transistor Tr₂ which is in the "off" state is removed, so that the plus voltage applied to the plus input terminal T₁ is impressed on the base of the second transistor Tr₂ through the resistors R₁, R₂ and R₄ to turn the second transistor "on". The first transistor Tr₁, accordingly, has its base grounded through the second transistor Tr₂ and turns "off", with the result that the voltage of the plus output terminal T₄ falls abruptly. A falling edge 3 shown in FIG. 5C involves a time delay under the influence of the stray capacitances of the first and second transistors Tr₁ and Tr₂ and the bias resistors. Since, however, the impedance of the drain of the FET element connected to the plus output terminal +V_(out) is small, about 2Ω, the aforementioned time delay is much smaller than a time delay at the minus output terminal -V_(out) to be described later. The falling edge 3 is therefore illustrated as having no time delay On the other hand, the side of the minus output terminal T₅ to which the gate electrode of the FET is connected has an impedance of 300Ω, which is 150 times greater than the impedance of the drain mentioned above. As shown by a rising part 4 in FIG. 5D, therefore, the voltage of the minus output terminal T₅ rises with a large time delay τ in accordance with a time constant which is determined by the impedance of the gate and the capacitance of the capacitor C₁ 2-3 μF.

In the interrupting operation, therefore, the FET has its drain side turned "off" a predetermined time earlier than its gate side, without fail. Accordingly, the FET is prevented from breaking down.

In FIG. 1, the Zener diode D₁, for setting the reference voltage, is disposed in order to prevent instability attributed to fluctuations in the bias voltage applied to the second transistor Tr₂, and it holds the potential between the node of the resistors R₁ and R₂ and ground at 10 V or so. The diode D₃ grounds the bias voltage of the plus input voltage +V_(in) along with the resistors R₁, R₂ and R₃, and upon application of the minus input voltage -V_(in), it also prevents the minus input voltage from being grounded.

Further, the diode D₂ prevents the plus voltage from outputting to the minus output terminal T₅ when the minus input voltage-Vin is not applied to the terminal T₂. The diode D₂ also prevents the discharge of the electric charge stored in the capacitor C₁ through the terminals T₃ and T₂ when the input voltage -V_(in) is at ground potential.

A second embodiment of the present invention will now be described in detail with reference to FIG. 2. The of difference is the addition of a pulse supplying circuit PS enclosed with a dotted line. The other elements are the same as in FIG. 1 and the same portions are assigned the same symbols and will not be explained again. A plus pulse voltage +V_(p) is impressed on the base of a third transistor Tr₃ through a resistor R₆. One end of a resistor R₇ is connected between the resistor R₆ and a pulse input terminal T₇, while the other end thereof is connected to the ground terminal T₆. The collector of the third transistor Tr₃ is connected to the base of the first transistor Tr₁, and the emitter thereof is grounded.

In the above arrangement, by way of example, a d.c. component of a waveform as shown in FIG. 6A is applied to the plus input terminal T₁, and a minus voltage as shown in FIG. 6B is applied to the minus input terminal T₂. Then, output voltages at the minus output terminal T₅ and the plus output terminal T₄ rise (1) and fall (2), respectively as illustrated in FIGS. 6D and 6E. At this time, the plus output voltage +V_(out) becomes steady state after the delay of a time interval τ.

Here, in the present invention, the pulse voltage +V_(p) is applied from the pulse input terminal T₇ as shown in FIG. 6C. Then, the third transistor Tr₃ has its base supplied with a plus voltage through the resistor R₆ and turns "on". The first transistor Tr₁ has its base grounded through the third transistor Tr₃ and is turned "off" state. As shown by waveform 5 in FIG. 6D, the plus output terminal T₄ provides substantially no voltage. The operation, in the case of interrupting the minus input voltage -V_(in), is the same as described with reference to FIG. 1, and is omitted from the description.

According to the arrangement shown in FIG. 2, as illustrated in FIGS. 6C and 6D, in the period of time during which the plus output voltage +V_(out) is unnecessary, the pulse input voltage +V_(p) is impressed so as to deliver no voltage to the plus output terminal T₄. Therefore, the efficiency of the power supply can be enhanced.

In FIG. 2, the resistor R₇ is connected for stability in the case where the pulse voltage +V_(p) is not impressed.

FIG. 3 shows a third embodiment of the present invention, in which a voltage regulator circuit VR, indicated by a dotted line, is added. More specifically, a series circuit comprising resistors R₉ and R₁₀ for voltage division a series circuit comprising resistor R₈, and a Zener diode D₄ for providing a reference voltage are respectively connected between the ground terminal and a line connecting the emitter of the first transistor Tr₁ and the plus output terminal T₄. The base of a fourth transistor Tr₄ is connected to the node of the resistors R₉ and R₁₀, and the emitter thereof is connected to a node of the resistor R₈ and the Zener diode D₄.

In the above arrangement, in the state in which the plus output voltage +V_(out) and the minus output voltage -V_(out) are delivered, the first transistor Tr₁ is in the "on" state and the base of the fourth transistor Tr₄ is supplied with a voltage which is determined by the voltage division ratio of the resistors R₉ and R₁₀. In the case where the divided output voltage is greater than the reference voltage of the Zener diode D₄, in excess of a predetermined value, the fourth transistor Tr₄ turns "on" and the first transistor Tr₁ turns into the "off" state, so that the plus output terminal T₄ is held at the predetermined voltage. As compared with the arrangement shown in FIG. 1, therefore, this embodiment can regulate the plus output voltage +V_(out).

FIG. 4 shows a fourth embodiment of the present invention, in which the pulse supplying circuit PS shown in FIG. 2 and the voltage regulator circuit VR shown in FIG. 3 are added to the circuit arrangement of FIG. 1.

The value of the resistors used in FIGS. 1A-4 are, for example, as follows: R₁ =150Ω, R₂ =1200Ω, R₃ =430Ω, R₄ =510Ω, R₅ =200Ω, R₆ =1KΩ, R₇ =1KΩ, R₈ =390Ω, R₉ =300Ω, R₁₀ =680Ω.

The transistors Tr₁ to Tr₄ may be formed by field effect transistors and then, the gate of the FET will correspond to the base of each of the transistors Tr₁ to Tr₄.

Since the present invention is constructed as described above, the destruction of an element is not incurred and errors in the operations of a drive means etc. can be prevented even when the sequence of applying or interrupting voltages from two voltage sources has been disordered.

Further, the efficiency can be enhanced by providing a constant output voltage only during the period of time when the voltage is necessary. The present invention has numerous merits. 

We claim:
 1. A power supply circuit, comprising:a first power source terminal operatively connected to receive a first voltage, said first voltage being a positive voltage; a second power source terminal operatively connected to receive a second voltage opposite in polarity to said first voltage applied to said first power source terminal; a reference terminal operatively connected to receive a reference voltage; a load operatively connected to said first power source terminal; a plurality of resistors, operatively connected in series between said first and second power source terminals; first and second one directional elements operatively connected to each other, said first one-directional element operatively connected to said second power source terminal and said second one-directional element operatively connected to said reference terminal; first and second output terminals, said second output terminal operatively connected to said second one directional element and said first output terminal operatively connected to said first power source terminal; a first switching element having a first control terminal, operatively connected in series with said first power source terminal; a second switching element having a second control terminal operatively connected to said plurality of resistors, operatively connected between said first control terminal of said first switching element and said reference terminal, said second switching element controlled by said first and second voltages applied to said first and second power source terminals, respectively, such that after said second voltage is applied to said second power source terminal, said second switching element provides a signal to said first control terminal for turning on said first switching element, such that after said second voltage is cut-off from said second power source terminal, said second switching element provides a signal to said first control terminal so as to turn off said first switching element before the voltage level of said second output terminal rises to the reference voltage, and such that when said second voltage is applied to said second output terminal before said first voltage is applied to said first output terminal, said first voltage is cut-off from said first output terminal before said second voltage is cut-off from said second output terminal independent of when said first and second voltages are applied or cut-off from said first and second power source terminals, respectively; anda capacitive element operatively connected between said second power source terminal and said reference terminal.
 2. A power supply circuit according to claim 1, wherein said first and second switching elements each comprise a transistor and wherein the reference potential is ground voltage.
 3. A power supply circuit according to claim 1, wherein the power supply circuit is operatively connectable to receive an external voltage, further comprising a third switching element having a third control terminal operatively connected to said first control terminal of said first switching element, thereby performing an ON or OFF operation of said first voltage from said first power source terminal by applying the external voltage to said third control terminal of said third switching element.
 4. A power supply circuit according to claim 1, further comprising an output terminal and a voltage regulator circuit, said voltage regulator circuit operatively connected in parallel with said first and second switching elements, for supplying a constant voltage to said output terminal.
 5. A power supply circuit according to claim 1, operatively connectable to receive an external voltage, further comprising:a pulse input terminal, operatively connected to receive the external voltage and operatively connected to said first switching element, for applying the external voltage to said first switching element; a third switching element operatively connected between said pulse input terminal and said first switching element; a first resistor operatively connected between said third switching element and said pulse input terminal; and a second resistor operatively connected between said pulse input terminal and ground.
 6. A power supply circuit according to claim 4, wherein said voltage regulator circuit, comprises:a third switching element operatively connected to said first switching element; a first resistor operatively connected to said third switching element and said output terminal; a second resistor operatively connected to said first resistor, said third switching element and ground, forming a voltage divider with said first resistor for supplying a voltage to said third switching element; a third resistor operatively connected to said first switching element and said third switching element; and a diode operatively connected between said third resistor and ground.
 7. A power supply circuit according to claim 6, operatively connectable to receive an external voltage, further comprising:a fourth switching element operatively connected to said third and first switching elements and ground; a pulse input terminal, operatively connected to receive the external voltage and operatively connected to said fourth switching element, for applying the external voltage to said fourth switching element; a fourth resistor operatively connected between said fourth switching element and said pulse input terminal; and a fifth resistor operatively connected to said pulse input terminal and ground.
 8. A power supply circuit, comprising:a first power source terminal operatively connected to a first voltage which is a positive voltage; a second power source terminal operatively connected to a second voltage opposite in polarity to said first voltage; a reference terminal operatively connected to a reference voltage; a load operatively connected to said first power source terminal; resistor means, operatively connected in series between said first and second power source terminals, for controlling the application of said first and second voltages; a first diode operatively connected to said second power source; a second diode operatively connected to said first diode and operatively connected between said reference terminal and said second power source; first and second output terminals, said second output terminal operatively connected to said second diode and said first output terminal operatively connected to said first power source terminal; first switching means having a first control terminal and operatively connected to said first power source terminal, for switching the voltage levels at said first output terminal; second switching means having a second control terminal operatively connected to said resistor means, operatively connected between said first control terminal of said first switching means and said reference terminal, for switching on and off said first switching means, said resistor means including means for applying said second voltage to said second power source terminal, for turning on said first switching means, said resistor means including means for cutting-off said second voltage from said second power source terminal, for turning off said first switching means before the voltage level of said second output terminal rises to the reference voltage, said resistor means including means for applying said first voltage to said first output terminal before said second voltage is applied to said second output terminal, independent of the order in which said first and second voltages are applied to or cut-off from said first and second power source terminals, respectively; and a capacitive element, operatively connected between said second power source terminal and said reference terminal, for providing a time delay to said second output terminal. 